1. Field of the Invention
This invention relates to nuclear reactor pressure vessel supports and, more particularly, to a link and pin support system that provides primary vertical and lateral support without restricting thermally induced radial expansion and contraction of the vessel.
2. Summary of the Prior Art
Nuclear reactor pressure vessels must be supported by structures that can adequately restrain vessel movement and accommodate the static, dynamic, and thermal loads which occur during normal operating conditions, in addition to the most adverse combination of loadings which may be experienced during postulated accidents and seismic events.
Design of the support structure will naturally be interrelated with various aspects of the reactor including its size and application, e.g., ship propulsion or electric power generation.
A number of support systems have been used in the prior art.
Primary vertical support has often been achieved by the utilization of cylindrical or frustoconical support skirts attached or integrally formed at the bottom of the reactor vessel. The skirt construction also permits radial growth of the vessel due to temperature and pressure through bending of the skirt in the manner of a vertical beam on a foundation. The skirt's length is chosen so as to permit this bending to take place safely. When space does not allow sufficient skirt flexing length, a construction consisting of a partially longitudinally slotted skirt can be used. The slotted portion acts as a multitude of cantilevers, while the unslotted portion, under the imposition of the moments and forces transmitted by the cantilevered portion, behaves as a cylinder. The support skirts rest upon soleplates, pedestals or the like.
Radially extending brackets circumferentially spaced about the reactor vessel or rings attached to its external surface which bear upon the horizontal surfaces of an enclosing reactor containment structure have also been used to achieve primary vertical support. The radial brackets and rings additionally have accommodated radial thermal growth displacements by the provision of means enabling siding contact to exist between the vessel and the containment. The reactor vessel support flange has similarly been used as a support means which bears upon portions of the containment structure.
Reactor vessels have been designed, moreover, which utilize the main coolant flow nozzles to perform the support function. In these cases, the nozzles may be arranged to transmit loads to the walls of a surrounding containment well. Typically, pads formed at the underside of the nozzles bear upon and are supported by wear plates that are disposed on the containment well walls. Guide channels and lubricants may be employed to facilitate radial movement. Where the containment well walls are concrete structures, cooling means may be required between the wear plates and walls to assure that the walls are not subjected to the high temperature of the coolant flowing through the nozzles. Alternately, vertical columns have been connected between the nozzles and a support base within the containment structure. In this arrangement, the columns are designed to accommodate relative displacements between the vessel and containment structure by flexing, thereby eliminating the need for relative sliding movements and allowing the columns to be securely fixed to the reactor vessel.
Support of reactor vessels at the nozzles requires that the nozzle structure have sufficient strength to accommodate the primary loads. Since nozzle sizing is generally dependent upon process conditions and the reactor's power rating, use of the nozzles for primary support necessitates additional nozzle reinforcement, strengthening and the like, not otherwise required. Strengthening of the nozzles to the extent necessary to carry design loads can be prohibitively expensive or otherwise impractical, particularly in reactor applications such as for marine propulsion.
The use of support brackets, moreover, are not generally considered on shipboard where high horizontal and vertical loadings, as well as roll and pitch, are involved.
A support structure which does not require strengthening of the nozzles or support brackets and allows unrestricted radial thermal growth without the need for sliding, lubricated guide channels and cooled support structures is desirable.